Electrochemical cell

ABSTRACT

The effective generation of electrical power is provided utilizing materials which are both chemically and electrochemically highly reactive with one another, in particular, alkali metals and water. The transport of water molecules to the alkali metal anode surface is restricted in the electrolyte. In a simple single stage cell an alkali metal hydroxide aqueous solution electrolyte is provided between an alkali metal anode and a non-reactive metal cathode at a sufficiently high concentration to prevent melting of the anode or thermal runaway. No other chemical or mechanical intermediary is required. Electrical power is obtained by continuously adding to the electrolyte during operation a controlled restricted quantity of water proportional to the electrical current withdrawn from the cell. No external heating is required and the cell operates near room temperature. The control of the rate of water addition may be used to control the power output of the cell.

United States Patent 11 1 Rowley 1 ELECTROCHEMICAL CELL [75 Inventor:Leroy S. Rowley, San Jose, Calif.

{73] Assignee: Lockheed Aircraft Corporation,

Burbank, Calif.

[22] Filed: Apr. 14, 1971 v [21] Appl. No.: 133,833

52 US. Cl f. 136/100 R [51] Int. Cl. H0lm 13/00 [58] Field of Search.136/100 R, 100 M, 86 A, 194, 136/83, 83 T [56] References Cited UNITEDSTATES PATENTS 267,319 11/1882 Berstein 136/100 R 3,554,810 1/1971Zaromb 136/86 A 2,921,110 1/1960 Crowley et al...,..... 136/100 RFOREIGN PATENTS OR APPLICATIONS 524,077 4/1956 Canada 136/154 PrimaryExaminer-Allen B. Curtis Attorney, Agent, or Firm Paul Morgan; George C.Sullivan [1 1 3,791,871 1451 Feb. '12, 1974 [57] ABSTRACT The effectivegeneration of electrical power is provided utilizing materials which areboth chemically and electrochemically highly reactive with one another,in particular, alkali metals and water. The transport of water moleculesto the alkali metal anode surface is restricted in the electrolyte. In asimple single stage cell an alkali metal hydroxide aqueous solutionelectrolyte is provided between an alkali metal anode and a non-reactivemetal cathode at a sufficiently high concentration to prevent melting ofthe anode or thermal runaway. No other chemical or mechanicalintermediary is required. Electrical power is obtained by continuouslyadding to the electrolyte during operation a controlled restrictedquantity of water proportional to the electrical current withdrawn fromthe cell. No external heating is required and the cell operates nearroom temperature. The control of the rate of water addition may be usedto control the power out- 15 Claims, 8 Drawing Figures PAIEN-IEBFEBIZIQM3.791.871

" sum 1' nr 3 FIG 2 FIG.4

ma? j ELECTROCHEMICAL CELL The present invention relates to an improvedmeans and method for generating electricity directly from theelectrochemical reaction of alkali metals with water in a simple cell'in a controlled alkali metal hydroxide electrolyte Fuel cell and batteryart has previously taught the limiting necessity of separators, spacers,membranes, porous barriers, dynamic films, mercury amalgams, al-

loys with less active metals, nonaqueous electrolytes or hightemperature molten salts between the cell electrodes to preventelectrical shorting and to prevent direct, violent chemical combinationswhere alkali metals were utilized. Examples are shown by US. Pats. Nos:1,015,734; 1,015,735; 2,605,297; 2,646,458; 3,014,084; 3,031,518;3,236,694; 3,449,165; 3,471,335; 3,488,221; and 3,507,703. Anotherrecent example is illustrated in a co-pending U. S. Patent applicationby Wilson S. Geisler, Ser. No. 8,606, filed Feb.- 4, 1970, entitledElectrochemical Energy Source, which utilizes moving mechanicalcomponents to provide a high output dynamic thin film alkali cell. Thesepatents demonstrate over 65 years of technical efforts to make moreeffective use of alkali metals in batteries.

The above-cited Pat. No. 2,605,297 teaches that ;..it is desirable tohave a highly electropositive material as an electrode. In the known artthese electropositive propertieshave been found only in alkali metalsand in alloys of alkali metals which are rapidly attached by water. Therapidity of such attack by water and the mechanical and physicalproperties such as strength and melting point has prevented thepractical use of these materials for many purposes for which they are,otherwise, well suited; for example, electrodes in water activatedprimary cells,

The above Pat. No. 1,015,734 filed in 1906 teaches that: Since metallicsodium is decomposed by water and aqueous solutions of caustic soda itcannot be brought directly into contact with an electrolyte. The aboverecent Pat. No. 3,507,703 issued Apr. 21, 1970 still teaches that ...thedirect single-step electrochemical oxidation of an alkali metal toobtain electrical energy is not feasible in an aqueous electrolytebecause of the rapid reaction of the alkali metal with water, ...thevigorous chemical reaction which occurs between an alkali metal andwater requires the use of nonaqueous electrolytes in contact with thealkali metal.

In contrast to the above teachings, the present invention permits thesafe and effective use of alkali metals with an aqueous electrolyte in asimple cell configuration which does not require moving parts, dynamicfilms or mechanical separators between the electrodes, or alloying oramalgamation of the alkali metals, and yet provides energy densities inthe range of 330 watt hours per kilogram of anode material for sodium,and over 2,000 for lithium. The only consumed materials are the anodemetal and water. The cell simplicity allows the total system weight tobe not substantially greater than that of these consumed materialsalone.

In the cell of the invention the anode can be entirely alkali metal. Theelectrolyte can bespontaneously locally generated by the reaction of thealkali metal anode with water to form an alkali metal hydroxidesolution. The ability to use water to provide for the batteryelectrolyte is an important advantage, as emphasized by the U. S.Supreme Court in United States v. Adams, 383 US 42; 148 U.S.P.Q. 479.Control of the electrolyte hydroxide concentration governs the rate ofreaction of the water in the electrolyte with the anode metal to preventanode melting and thermal runaway. The oxidizer can be untreated freshor sea water or any other suitable aqueous solution. It is addedcontinuously at an appropriate restricted rate to the electrolyte whileelectrical power is simultaneously drawn from the cell. The rate of thiswater addition is controlled to replace the water reduced and alsomaintain the electrolyte concentration balance. The cathode, at whichthe added water is electrochemically reduced, can be any suitablenon-reactive conductive metal surface spaced from the anode. Thus, inthe cell of the invention, electricity is generated safely andeffectively from a highly active solid alkali metal anode and a simplercathode, both directly immersed in an appropriate, controlled aqueouselectrolyte into which water is fed at an appropriate restricted rate.

The present invention allows the direct use in batteries of alkalimetals which are good electrical and thermal conductors, lightweight,widely available, inexpensive, and can provide high energy densities. ltallows their use with the available water in low cost, mechanicallysimple structures having good energy outputs per unit weight.

Further objects, features and advantages of the invention pertain toparticular arrangements, structures and operations whereby theabove-mentioned aspects of the invention are attained. The inventionwill be better understood by reference to the following description andto the drawings forming a part thereof, wherein:

FlG. l is an axial cross-sectional view of a first embodiment of anelectrochemical cell in accordance with the present invention;

FIG. 2 is a cross-sectional view of the embodiment of FIG. 1 taken alongthe line 22 of FIG. 1;

FlG. 3 is a perspective view of a second embodiment of the presentinvention;

FIG. 4 is an enlarged partial cross-sectional view of the embodiment ofFIG. 3 taken along the line 4-4 of FIG. 5 is an axially cross-sectionedperspective view of a third embodiment of the present invention;

FIG. 6 is a cross-sectional view of a fourth embodiment of the presentinvention;

FIG. 7 is an axial cross-sectional view of a fifth embodiment of thepresent invention, fully submerged in water; and

FIG. 8 is a diagram showing the operating region for a sodium anode andsodium hydroxide solution electrolyte.

Referring first to FIGS. 1 and 2, there is shown therein a firstexemplary battery cell 10 in accordance with the present invention. Thediscussion hereinbelow of the cell 10 of FIGS. 1 and 2 is alsoapplicable to the subsequently described exemplary batteries or cells50,

i 100, and 200 of FIGS. 3, 4, 5, 6, and 7 respectively,

except as to described differences.

The cell 10 includes a central alkali metal (sodium) anode cylinder 12surrounded by an'insulator/spacer of a thin film 14 of non-reactive,water soluble material which is not an electronic conductor. A preferredmaterial for the film 14 is the natural hydrated oxide which forms onthe sodium surface when -it is exposed to humid air. Other suitablewater soluble electrical insulators may be used for the film 14.Directly surrounding this film 14, and providing the cathode, is anonreactive electrically conducting cylindrical tube 16, spaced from thesodium anode by the film 14. The cathode tube 16 may be covered with anouter envelope 18 of partially thermally-insulating material, such asplastic, if desired for certain applications.

' Embedded in the sodium cylinder 12 is an electrical contact 20 whichis fed through and sealed in a bottom insulating cap 22 to form thenegative anode terminal of the battery. It is important to make a goodelectrical contact 20 with the sodium, preferably solidly embedding thecontact wire into the sodium. The insulating base cap 22 here can bemade of molded plastic or else the end of the cell 10 may be dipped in asuitable potting compound. The positive cathode terminal 24 is formed byany suitable electrical connection to the cathode tube 16.

' An upper plastic cap 26 on the battery is apertured for the inlet ofwater 28 through a capillary aperture 30 and for hydrogen gas escapethrough a vent hole 32. A long shelf life may be obtained if' the venthole 32 is plugged with an only slightly permeable removable plug 38such as cork or wood. A valve 34 is provided for admitting the water 28into the cell at a controlled.

rate from a reservoir 36.

Referring to the film 14, when the natural hydrated oxide of sodium isto be employed it suffices to oxidize the sodium surface for minutes inair. The surfaceoxidize sodium cylinder 12 may then be inserted into arigid concentric containing tube 16 to form a snug, but not right, fitso that there is no metalic electrical contact between the sodium andthe tube 16. Alternatively, the tube 16 may be formed around thesurfaceoxidized sodium cylinder 12 by rolling metal foil around thecylinder 12 and applying a suitable sealer at the seam, for example, anepoxy, which, if applied completely around the tube 16, can also serveas the thermal insulator 18 and the cell container.

A space at least one-half the volume of the sodium 12, plus a smalladditional distance is provided between the top of the sodium and thebottom of cap 26 initially to allow for the reaction product 40(hydrated sodium hydroxide) to accumulate and also to afford some smallgas space below the cap 26. Alternatively, an aperture in the side ofthe cell may be provided for excess electrolyte removal during the cellsoperation.

When the battery is operated, the water 28 is admitted through the smallaperture 30 by the valve 34 at a slow controlled rate which must bematched to the thermal and electrical loads on the battery in order tokeep the internal operating temperature within appropriate limits. Theaqueous solution 28 may be fed from the reservoir (container) 36 or theentire battery may be submerged in fresh water or sea water. It will beappreciated that the valve 34 may be eliminated and the propercontrolled rate of water input may be achieved through use of anappropriately sized capillary tube 30 The plug 38 may be removedautomatically by the action of internally generated hydrogen gaspressure in the following manner. The valve 34 is opened briefly toadmit a few drops of water and is then closed. The hydrogen generatedinternally by the reaction will push out the plug 38. Subsequent openingof thevalve 34 by an appropriate amount will assure operation of thebattery. Alternatively, the plug 38 (if initially present) may beremoved by hand and the proper setting of valve 34 made prior to fillingof the reservoir.36 or submersion of the battery. Various conventionalsimple springloaded automatic valves may-be used in place of the plug38. i

The battery diameter, length, thickness of thermal insulation 18, valve34 setting and/or capillary 30 diameter can be varied to provide for theintended use in terms of electrical load, duration of discharge andintended environment. If the electrical power demand is always less thanthe maximum designed rating of the battery in the design environmentthen considerable flexibility is afforded through use of the valve 34.An appropriate water inlet rate may be readily established empiricallywhich will result in a satisfactory internal operating temperature, andelectricity (at rates below the maximum power rating) may-then be drawnat the level needed, albeit at varying overall energy conversionefficiencies.

Tube 16 internal diameters of between 0.5 and 6.0 centimeters have beenoperated satisfactorily and there seems to be no limitation on the useof larger battery diameters, with proper design. With a tube 16 internaldiameter of 1.27 cm., in operation in room temperature air with nothermal insulation 18, a maximum steadystate power output of 2 watts wasachieved. This observed power output was nearly independent of batterylength, provided the length exceeded several centimeters.

A critical design consideration is to limit the maximum rate at whichwater can enter the cell. Flooding of the battery with water will causea thermal runaway, and usually an explosion. Sufficient excess water inthe electrolyte will cause a rise in internal temperature above 975C,which is sufficient to melt the sodium mass. When the sodium melts, animmediate increase in its reaction rate with the electrolyte occurs,resulting in an explosion. On the other hand, too great a heat loss fromthe battery, as when the battery is without any thermal insulation 18and is immersed in ambient temperature water or other cooling liquid,results in excessively low electrical power output becausethe internaltemperature remains below that necessary to liquify the sodium hydroxideelectrolyte. For these reasons the effective thermal conductivity of thebattery walls must be tailored to the environment of application and thedesired power output.

Considering the apparent scientific theory of operation involved, it isknown that water first enters the battery and dissolves the upperportions of the soluble film l4 and makes some hydrated sodium hydroxideplus heat. While the temperature is low the product formed is a solidwith very low mobility of water molecules and ions. When the temperaturebecomes sufficient to liquify the hydrated sodium hydroxide, and therebyprovide a liquid electrolyte, useful electrical power is obtained. It isbelieved that the water molecules directly adjacent to the sodiumsurface at any instant are immediately consumed by direct chemicalaction, thereby establishing a protective film and a concentrationgradiant (concentration polarization) across which water molecules mustdiffuse before further chemical reaction can occur at thesodium/electrolyte interface. However, water molecules at the cathode 16can be reduced electrochemically to hydrogen gas and hydroxide ions bythe electrons supplied from the sodium through a closed externalcircuit. The high mobility of the hydroxide ions produced near thecathode favors their migration away from their point of origin andpresents less restriction to the continued migration of water moleculestoward the cathode than toward the polarized anode. There is noelectrochemical requirement for water molecules to reach the anodesurface,

only hydroxide ions, because the anode metal ions can readily enter theelectrolyte, even at a water-starved anode/electrolyte interface.Therefore, the combination of: (1 the diffusion barrier for water at theanode; (2) the provision for electron transfer at high rates from theanode to the cathode through a low impedence external circuit; (3) thelow ohmic resistance of the electrolyte; and (4) the restricted rate ofwater addition to the electrolyte all make possible a continuousselective favored migration of the water molecules towards the cathode.This makes possible the realization of high coulombic efficiencieswithout recourse to selective porous barriers, membranes or the like.

Therefore, it may be seen that without the insertion of barriers, whensufficient high current densities are drawn from the cell to lower itsvoltage substantially below the open circuit voltage,.preferentialdiffusion of water to the cathode is favored. For example, in typicalexperiments the coulombic efficiency was percent at a cell voltage of 1volt under load, but increased to above 60 percent at 54; volt. Thus,over half of the water molecules involved in the chemical andelectrochemical (versus hydration) reactions in the cell were beingreduced at the cathode when high currents (at low voltages) were drawn.

The relationship of internal temperature to the functional factors givenabove is best understood in terms of electrolyte composition. For thispurpose, for a sodium anode cell the phase diagram for sodium hydroxideand water given in Mellors standard text A Comprehensive Treatise onInorganic and Theoretical Chemistry, Volume 2, Supplement 2, page 502(1961) is most helpful. These data are provided in FIG. 8, where theoperating region is indicated by the shaded area 300 with the preferredoperating point 302 located therein. From this phase diagram it isreadily seen that between the temperatures of 50C and 97C a homogenousliquid exists between sodium hydroxide and water for concentrations asgreat as 60 percent by weight sodium hydroxide at 50C, with higherconcentrations as the temperature increases above 50C. The preferredconcentration of 70 percent by weight sodium hydroxide is a solutionabove 65C. This is approximately 30.

molar sodium hydroxide and corresponds to the liquid sodium hydroxidemonohydrate.

The shaded area 300 in FIG. 8 indicates the range of steady-stateelectrolyte compositions which can provide adequately stable operatingconditions-for cells utilizing sodium anodes. As may be seen, the area300 is roughly triangular, being bounded on the left side by a lineestablished empirically from determinations of the maximum amounts ofwater safely allowable in the electrolyte at various temperatures. Thearea 300 approaches an upper boundary set by the melting temper ature ofsodium. On the right, area 300 approaches the liquid/solid boundary.This boundary is formed by the liquid/solid curve for sodium hydroxidemonohydrate (NaOH'H O) and the liquid/solid line for sodium hydroxide(NaOl-l). It is to be understood that the closeness of this approach tothe liquid/solid limiting boundary is constrained by two requirements.The first of these requirements is that solidification of theelectrolyte is to be prevented during operation of the cell. Inexperiments it was observed, for example, with a battery operating witha liquid electrolyte composition of 70 percent by weight sodiumhydroxide, that when the internal temperature was allowed to drop belowC there was a sharp increase in the internal resistance of the battery,because the electrolyte solidified, conforming to FIG. 8. The second ofthe above two requirements is that sufficient water molecules must bepresent in the electrolyteto support the cathode partial reaction, asrequired for a net electrical power output from the cell.

Therefore, the electrolyte must remain at a temperature below themelting point of the alkali metal anode and not in excess of the boilingpoint of the electrolyte. Furthermore, for the sodium/water system, itmay be seen that the electrolyte composition operating regimecorresponds approximately to the composition of the liquid sodiumhydroxide monohydrate which is percent sodium hydroxide by weight.

Pertaining to the amount of water which must be added to satisfystoichiometric requirements of the reactions involved in operating thebattery, it is readily seen that approximately two water molecules areneeded for each atom of sodium consumed. One of these water molecules isrequired to transform sodium to sodium hydroxide (whether by directchemical or by electrochemical reaction), and the second water moleculeis necessary to form the monohydrate of the sodium hydroxide for thereasons given above.

As one practical way of determining the maximum acceptable cell waterinlet rate, it is sufficient to measure the volume evolution rate ofhydrogen from an operating cell. From this measurement the number ofmoles of hydrogen evolved per second is readily calcu lated. The maximumnumber of moles of water needed per second is just four times the numberof moles/sec of molecular hydrogen evolved. Water added in excess ofthis rate will either raise the temperature to the danger point or, ifsufficient cell or anode cooling is available to hold the temperaturebelow 97C, then the sodium hydroxide concentration will drop, due todissolved but unreacted water accumulation in the electrolyte, until thereaction rate and heating eventually exceed the cooling capacity. Ineither case an explosion is the likely result.

When the water inlet is closed, excess water in the electrolyte reactswith sodium at a rate which tends to decrease with the water content.Correspondingly, the

heat evolution rate decreases and the internal temperature drops untilthe electrolyte solidifies. At this point the reaction stops forpractical purposes and the battery is inactive until water is added.Thus the system is easily regulated by the water supply rate, providedthat this rate remains below the above-discussed maximum limitations.

Accordingly, the sodium cell 10 is self-protecting and soon becomesinactive when no further water is allowed to enter the cell. Theelectrolyte solidifies, and no significant internal reaction is thenpossible until more water is added from an external source. Yet thesystem may be activated simply by adding water, which migratessufficiently to start the reactions and generate heat. Activation islargely independent of the ambient temperature, and no activatingchemicals or processing are required. Any and all heat required toachieve the desired operating condition may be provided simply by theexothermic reaction in the cell. Thus, low ambient temperature and thepresence of solid electrolyte in the cell does not hinder itsactivation, and the cell quickly operates stably at near-roomtemperatures. Cells can be designed for a start-up time of a few secondsor less if fast start-up is desired.

In summary, the necessity for achieving a set of interrelated conditionsfor useful, efficient and safe operation of a sodium/water system as abattery is apparent from the discussion above. The structure and methodof the invention satisfies these interrelated conditions to achieveuseful electrical power extraction from an otherwise violent reaction ofan alkali metal and water. Rather than reducing the activity of thealkalimetal anode by amalgamating it or alloying it with a highpercentage of a less active metal (as was previously believed necessaryin order to directly engage the alkali metal anode with an aqueouselectrolyte without thermal runaway), the present cell enables thedirect utilization and total immersion of the active solid alkali metalitself as an anode, by limiting the activity of the aqueous electrolyte.This limitation of the electrolyte activity is accomplished throughrestriction of the rate at which water molecules can arrive at the anodemetallic surface. A high electrical power output is provided 'by thecontinuous introduction of only a restricted quantity of water duringthe operation at a rate which maintains the electrolyte hydroxideconcentration and replaces the water consummed. The water required isthe water reduced electrochemically at the cathode, plus the waterconsummed by direct chemical reaction with the anode, plus thehydration/dilution requirement to meet the increases in the amount ofhydroxide in the electrolyte.

Referring next to FIGS. 3 and 4, there is shown therein a secondexemplary battery 50 in accordance with the present invention. FIG. 4 isa cross-section taken from FIG. 3 along the direction 4-4. The cell 50has a spiral configuration. It consists of a sheet of alkali metal(sodium) anode 52, a thin electrically insulating film of solublematerial 54 (such as the oxide layer described previously), a sheetmetal cathode 56, a terminal 58 embedded in the alkali metal anodematerial 52, a lower plastic cap or potting compound seal 60, a cathodeterminal 62, a water supply means 64 for supplying water to theelectrolyte at .a controlled rate through valve 66, an electrolyte 68,and suitable vertical end seals such as the inner seal 70 shown toretain the electrolyte in the cell 50.

The exemplary closed spiral configuration of the cell 50 illustrates theuse of both sides of a sheet metal surface as part of the cathode 56area. Thus, the two active surfaces of the cathode 56 shown in FIG. 4represent two turns in the same continuous sheet of metal.

It will be understood that as an alternative to the single continuouscathode arrangement shown in FIGS. 3 and 4 that two separateparallelsheet metal cathodes may be placed on opposite sides of the anode 52, toform a flat plate construction, or to be similarly rolled up in the formof an open or closed spiral. This form requires more space and materialbut provides increased external cathode surfaces availablefor externalcooling and also lends itself to the series connection of cells forvoltage addition.

Advantages afforded by thin flat plate or spiral cell constructions, ofwhich the embodiment 50 is exemplary, are their compactness and ease ofconstruction and assembly. For example, a sheet of sodium may be easilyrolled out as a continuous ribbon of the desired thickness, air oxidizedas described previously, and then laid on, and rolled up within, a thinmetal sheet to form the body of the battery as shown in FIG. 3.

Referring next to FIG. 5, there is shown therein a third exemplarybattery 100 in accordance with the present invention. The cell 100 hereconsists of a central cylindrical solid sodium metal 'anode 102surrounded by an annular space 103 between the anode and an enclosingcylindrical cathode wall container 106. This space 103 is preferablyinitially charged with a water-depleted electrolyte 104 which is solidsodium hydroxide monohydrate. Provision is made for thermal insulation108 as needed, and for a bottom seal 110. The cathode is contacted bythe positive terminal 112, while the anode 102 contains a suitablyembedded negative terminal 114. Water is supplied at a controlled rateby gravity feed from a container 116 through a valve 1 18 down through atube extending down into the electrolyte to near the bottom of the cell.The anode 102 is considerably longer than the depth of the container 106here in order to provide a continuous gravity feed of anode materialinto the cell as the anode metal is consummed by the cell reactions. Aninsulating collar and support mechanism 120 is therefor provided to holdand guide the anode metal down into the center of the cell.

A principal advantage of the configuration exemplified by the cell 100is the versatility afforded by the use of large diameter anodes withvariable anode feed rates and with relatively gross spacing between theanode metal and the cathode compartment walls. A s excess electrolyteforms during operation, it can run over the top of the cathode wall 106,or leave through an appropriate exit port provided in the side wall ofthe container, or be removed and stored in containers provided for itsaccumulation. The anode metal feed rate is slow, and the cell operatesin the same general manner as the other disclosed static cells.

As an example of the configuration exemplified by the cell 100, it wasfound convenient in an experiment to use a standard commercialcylindrical billet of sodium (0.45 kg., 5.8 cm. diameter) taken directlyfrom its shipping can as the anode 102, with a cathode container 106consisting of a 6.0 centimeter internal diameter stainless steel beaker(321 alloy) 14 centimeters deep to support and guide the sodium billet,and a small (1 millimeter inside diameter) water feed tube of plasticinserted down the side wall in the space 103 to approximately the bottomof the stainless steel beaker. The electrolyte composition was held atapproximately percent by weight sodium hydroxide through a controlledwater input rate of 0.5 milliliters per minute. For this test theelectrolyte temperature was maintained at approximately C by a watercooling coil soldered on the bottom exterior surface of the beaker 106.With this simple cell, 130 grams of sodium anode was consumed whilesimultaneously withdrawing electrical power at an average rate of 3.0watts at 0.6 volt in a 400 minute test run with a constant anodeimmersion depth of only 1.0 centimeter in the electrolyte, maintained bytwo overflow holes in the side of the beaker one centimeter above thebottom.

Lithium was used as the anode material in a similar test apparatus whichwas immersed in a constant temperature bath to provide a desiredtemperature level control of the electrolyte. A cylindrical lithiumanode, l centimeter in diameter, was immersed to a constant depth of 2centimeters in the electrolyte, said constant depth being maintained byoverflow of excess electrolyte as it was produced in the cell. The cellwas operated at 1 volt, with a water input rate of 1 milliliter perminute for the following steady state values: with the electrolytetemperature maintained at C and a power output of 1 watt, 5.3 miligramper minute oflithium was consummed, and the electrolyte wasapproximately 0.75 molar lithium hydroxide; with the electrolytetemperature maintained at 60C and a power output of 1.2 watt, l0miligram per minute of lithium was consummed with an averageelectrolyteconcentration of approximately 1.5 molar lithium hydroxide.

It is apparent that the rate of reaction between lithium and diluteaqueous solutions is much lower than is the casewith sodium; Thetenacious surface film of lithium hydroxide which forms on the lithiumanode is presumably more effective inreducing the flux of watermolecules to the anode than is the case with sodium. Consequently, incontrast to sodium, lithium can achieve relatively high efficiencies (interms of watt hours per pound of anode metal consummed) when immersed inrelatively more dilute aqueous solutions than sodium without melting,due to a higher point of lithium and its less'soluble surface film. As aresult, the vol ume of electrolyte produced per unit volume of lithiumconsummed in this operating mode is much larger than the 50 percentincrease in electrolyte volume associated with the conversion of sodiumto the sodium hydroxide monohydrate electrolyte described above. Fromthe values given above for lithium, for example, it is seen that 1 cubiccentimeter of lithium consummed at 15C results in a solution of 0.75molar lithium hy droxide whose volume is approximately 0.1 liter, or a100 fold volume increase to maintain the same solution. The electrolytecan be somewhat more dilute than 0.75 molar in lithium hydroxide but notso dilute that the electrolyte ohmic resistance becomes a seriouslimitation on the cell power. An absolute upper limit on electrolyteconcentration is the saturation limit of solubility in water, which forlithium hydroxide is known from the literature to be far below that forsodium hydroxide. As a practical matter an electrolyte concentra' tionwell below the hydroxide solubility limit is desirable to assureadequate transport rates of water molecules to the cathode. Thus forlithium, means are preferably provided for hydroxide solution removalfrom the cell during its operation as discussed previously. Also, it canbe appreciated that continued exposure of lithium to a diluteelectrolyte in the cell will continue to consume the lithium even afterthe water feed is stopped, as the water is in considerablestoictiometric excess, in contrast to sodium cells withtheir highmolarity electrolytes. Therefore, in the case of lithium, whenelectrical power is no longer needed the electrolyteor the anode shouldbe removed from the cell if the remaining lithium is to be saved forfuture use.

Water can be reclaimed from the aqueous lithium hydroxide electrolyteproduct, for example, by bubbling carbon dioxide therethrough andremoving through precipitation the resulting insoluble lithiumcarbonate. Various uses can be envisioned for this well known propertyof lithium to trap carbon dioxide and to form an insoluble carbonate.

For other active alkali metal anode metals, including alloys, it will beappreciated that those skilled in the art, using the teachings providedherein, can determine their appropriate electrolyte concentrations,water feed rates and operating temperatures by careful routineexperimentation.

Cell cooling means can be provided in a variety of ways besides the aircooling and water immersion cooling shown. For example, an ethyl alcoholreservoir will control the temperature at essentially 78C, its boilingpoint. An alcohol heat pipe in good thermal contact with the anode maybe employed as another means for cooling and stabilizing the celloperating temperature.

It will also be appreciated that a suitable gas trap around the cell,comprising a sliding seal, double wall enclosure, or other suitableconventional arrangements, may be used for collecting and utilizing thehydrogen liberated by the cell reactions. The evolved hydrogen is auseful product which, for example, can be directly used in fuel cells orengines to provide substantial additional power and useable water ifdesired. Alternatively, the hydrogen can be directly immediatelydisposed of in a simple conventional platinum catalytic burner. The heattherefrom can be used for a steam engine or other purposes. The evolvedhydrogen can also be consummed electrochemically in the cell by asuitable oxidizing agent, if desired.

individual electrode polarization measurements conventionally taken witha suitable reference electrode inserted in the electrolyte between theanode and the cathode have shown that essentially all cell polarizationoccurs at the anode. Thus, the depth of immersion of the anode in theelectrolyte can serve as a control over cell activity and output power.The evolved hydrogen may be used to control the anode immersion depth orelectrolyte level to provide a self-regulating power mechanism. Forexample, a sealed bellows arrangement pressurized by the evolvedhydrogen, linked to the anode and provided with a bleed valve can couplethe hydrogen evolution rate to the extension of the bellows andtherefore to the anode immersion depth which, in turn, will control thepower output of the cell. The same or a similar arrangement may be usedto control the water supply rate, that is, extension of the bellows maycontrol the degree of opening of the water valve.

Referring next to FIG. 6 there is shown therein a fourth exemplarybattery in accordance with the present invention. The cell 150 consistsof a sodium anode 152, a layer of soluble material 154, such as thealkali metal hydrated oxide formed in air described previously, anopen-mesh metallic screen 156 which is in contact with soluble material154 but which does not make metallic contact with the anode 152, a watersupply means 158 with a control valve 160, a porous material or sponge162 in contact with the metal screen 156, a positive cathode contact 164to the metal screen 156,

and a suitable anode contact 166 forming the negative terminal of thecell.

The sponge'material 162 is unnecessary to distribute the water uniformlyover the'metalscreen 156 when a capillary wicking type screen is used.In that case it suffices to drip or wick water anywhere onto the screenat a limited rate.

An advantage of the screen cell 150 embodiment is the facility itprovides for hydrogen escape and for vari ous uncritical rate means ofwater supply, for example mist or steam or fog in contact with the largeworking area exposed by the screen cathode. The cell 150 can operate atlow power densities solely from humid air such as is present in jungleatmospheres. In such cases a separate water supply means is unnecessary.

Referring next to FlG. 7, there is shown therein a fifth exemplarybattery 200 in accordance with the present invention/The cell 200consists of an alkali metal (sodium) anode 202 surrounded initially by athin f lm of soluble material 204 as described previously, a porousmetal jacket 206 which serves as acathode, a porous thermal insulationjacket 208, an insulating base cap 210 of molded plastic or pottingcompound, an embedded anode terminal 214, a positive cathode terminalconnection 216, an electrolyte 218 which forms during cell operation, agas space 220 to accommodate internal volumetric expansion associatedwith product formation, and a top cap 212 of plastic or any suitablesealing material.

The primary difference between this cell 200 of FIG. 7 and the cell 50of FIG. 1 is the use in the cell 200 of porous wall materials whosepurpose is to provide automatically controlled ingress of water from anunderwater environment into the cell, and automatic gas egress. The cell200 is intended primarily for submerged or immersed operation for use inoceans, lakes, rivers, etc.

It has been found that the porosity of the wall and jacket materials andthe thickness of the thermal insulating jacket can be adjusted toprovide satisfactory operationin a fully submerged condition withhydrogen gas leaving the cell through the porous walls as it is producedby the reactions. A further advantage for submerged operation of thecell 200 is its relative insensitivity to orientation. A furthermodification may be provided, namely, addition of a vent in cap 212which facilitates the escape of hydrogen during operation. The selectionof an appropriate diameter for the gas vent will prevent entrance ofwater therethrough because the capillary forces in the porous walls aregreater than in the vent tube. This has the effect of insuringpreferential entrance of liquid water through the porous walls and exitof hydrogen gas through the vent tube.

Additional advantages accrue from the use of porous metal walls 206. Forexample, it is unnecessary to provide the film 204 in advance in thiscase, because the initialwater attack through the side walls willconsume the alkali metal at the interface and isolate the anode 202electronically from the cathode, even if internal shorting is initiallypresent.

A long shelf life may be obtained through application of an externalwater-soluble coating or package around the cell or by storage in aremovable hermetic plastic bag.

It may be seen that there has been disclosed herein a novelelectrochemical cell. It is contemplated that numerous variations andmodifications may be made therein by those skilled in the art. Thefollowing claims are intended to cover all such variations andmodifications as fall within the true'spirit and scope of the invention.

What is claimed is:

1. An electrochemical cell comprising:

a solid elemental alkali metal consummable anode which is highlyreactive with water;

a non-reactive electrically conductive cathode spaced from said anode;

a container of liquid electrolyte in which said alkali metal anode andsaid cathode are commonly directly immersed as a static cell;

said cell providing an open and unimpeded liquid electrolyte circulationpath between said anode and said cathode; circuit connection means atsaid cathode and said anode for drawing electrical power from said cell;

said electrolyte in the operation of said cell consisting essentially ofa liquid solution in water of the hydroxides of said alkali metal anode;

said electrolyte having a liquification temperature substantially belowthe melting point of said alkali metal anode;

said hydroxide solution of said electrolyte being'of sufficiently highconcentration to restrict the direct chemical reaction rate between saidalkali metal anode and the water in said electrolyte at said alv kalimetal anode to prevent both melting of said anode and boiling away ofsaid electrolyte;

and electrolyte molarity control means for continuously controlling saidhydroxide solution concentration of said electrolyte as said cell isoperated to maintain said electrolyte at a temperature below the meltingpoint of said alkali metal anode and not in excess of the boiling pointof said electrolyte.

2. The electrochemical cell of claim 1 wherein said control meanscomprises water feed control means for continuously feeding additionalwater into said electrolyte at a controlled restricted rate in a mannerwhereby a portion of said water diffuses through said electrolyte tosaid cathode to be reduced electrochemically at said cathode aselectrical power is drawn from said cell by said circuit connectionmeans.

3. The electrochemical cell of claim 1 wherein said anode is sodium andsaid alkali metal hydroxide is sodium hydroxide and wherein saidsolution thereof corresponds approximately to the composition of sodiumhydroxide monohydrate.

4. The electrochemical cell of claim 1 wherein said anode is sodium andsaid alkali metal hydroxide is sodium hydroxide and wherein saidsolution is approximately percent sodium hydroxide by weight.

5. The electrochemical cell of claim 2 wherein said anode is lithium andsaid alkali metal hydroxide is lithium hydroxide.

6,' The electrochemical cell of claim 1 wherein said water feed meanscomprises a water supply means and water flow restricting aperture meansbetween said water supply means and said electrolyte.

7. The electrochemical cell of claim 1 wherein said anode and saidelectrolyte are enclosed in a porouswalled container providingrestricted water ingress and restricted hydrogen gas egress.

8. A method of generating electricity in an electrochemical cellcomprising:

directly commonly immersing a solid alkali metal anode highly reactivewith water and a nonreactive cathode in an electrolyte consistingessentially of a liquid solution in water of the hydroxides of saidalkali metal anode;

maintaining said electrolyte at an operating temperature above itsliquification temperature and below the melting point of said alkalimetal anode and not in excess of the boiling temperature of saidelectrol'yte;

maintaining said hydroxide solution of said electrolyte at asufficiently high concentration to prevent melting of said anode by thedirect chemical reaction between said anode and the water in saidelectrolyte;

and continuously adding water to said electrolyte at a restricted ratewhich maintains said electrolyte hydroxide solution and replaces thewater consummed in the cell by the chemical and electrochemicalreactions therein while electrical power is simultaneously withdrawnfrom between said anode and said cathode.

9. The method of claim 8 wherein said anode is sodium and saidelectrolyte hydroxide is sodium hydroxide and wherein said operatingtemperature is maintained above l2.3C and below 97.5C and said sodiumhydroxide electrolyte solution is maintained above'52 and below 75percent by weight.

10. The method of claim 8 wherein said anode is sodium and saidelectrolyte hydroxide is sodium hydroxide and wherein said operatingtemperature is maintained above 65C and below 975C and said sodiumhydroxide electrolyte solution is maintained at approximately 70 percentby weight.

11. The method of claim 8 and including adding water to said electrolyteby passing water through a porous-walled container at a restricted rate.

12. An electrochemical cell comprising:

a. a solid elemental alkali metal consummable anode which is higlyreactive with water;

b. a non-reactive electrically conductive cathode spaced from saidanode;

c. an electrolyte in which said alkali metal anode and said cathode arecommonly directly immersed as a static cell;

d. said cathode being shaped so as to contain said electrolyte;

c. said cell providing an open and unimpeded liquid electrolytecirculation path between said anode and said cathode;

f. circuit connection means at said cathode and said anode for drawingelectrical power from said cell;

g. said electrolyte in the operation of said cell consist ingessentially of a liquid solution in water of the hydroxides of saidalkali metal anode;

h. said electrolyte having a liquification temperature substantiallybelowthe melting point of said alkali metal anode;

i. said hydroxide solution of said electrolyte being of sufficientlyhigh concentration to restrict the direct chemical reaction rate betweensaid alkali metal anode and the water in said electrolyte at said alkalimetal anode to prevent both melting of said anode and blowing away ofsaid electrolyte; and

j electrolyte molarity control means for continuously controlling saidhydroxide solution concentration of said electrolyte as said cell isoperated to maintain said electrolyte at a temperature below the meltingpoint of said alkali metal anode and not in excess of the boiling pointof said electrolyte.

13. The electrochemical cell of claim 12 wherein said control meanscomprises water feed control means for continuously feeding additionalwater into said electrolyte at a controlled restricted rate in a mannerwhereby a portion of said water diffuses through said electrolyte tosaid cathode to be reduced electrochemically at said cathode aselectrical power is drawn from said cell by said circuit connectionmeans.

14. The electrochemical cell of claim 13 wherein said anode and saidcathode each have a substantially spiral configuration so that saidspiral anode is positioned within said spiral cathode.

15. The electrochemical cell of claim 7 wherein said cathode is anopen-mesh metallic screen.

2. The electrochemical cell of claim 1 wherein said control meanscomprises water feed control means for continuously feeding additionalwater into said electrolyte at a controlled restricted rate in a mannerwhereby a portion of said water diffuses through said electrolyte tosaid cathode to be reduced electrochemically at said cathode aselectrical power is drawn from said cell by said circuit connectionmeans.
 3. The electrochemical cell of claim 1 wherein said anode issodium and said alkali metal hydroxide is sodium hydroxide and whereinsaid solution thereof corresponds approximately to the composition ofsodium hydroxide monohydrate.
 4. The electrochemical cell of claim 1wherein said anode is sodium and said alkali metal hydroxide is sodiumhydroxide and wherein said solution is approximately 70 percent sodiumhydroxide by weight.
 5. The electrochemical cell of claim 2 wherein saidanode is lithium and said alkali metal hydroxide is lithium hydroxide.6. The electrochemical cell of claim 1 wherein said water feed meanscomprises a water supply means and water flow restricting aperture meansbetween said water supply means and said electrolyte.
 7. Theelectrochemical cell of claim 1 wherein said anode and said electrolyteare enclosed in a porous-walled container providing restricted wateringress and restricted hydrogen gas egress.
 8. A method of generatingelectricity in an electrochemical cell comprising: directly commonlyimmersing a solid alkali metal anode highly reactive with water and anon-reactive cathode in an electrolyte consisting essentially of aliquid solution in water of the hydroxides of said alkali metal anode;maintaining said electrolyte at an operating temperature above itsliquification temperature and below the melting point of said alkalimetal anode and not in excess of the boiling temperature of saidelectrolyte; maintaining said hydroxide solution of said electrolyte ata sufficiently high concentration to prevent melting of said anode bythe direct chemical reaction between said anode and the water in saidelectrolyte; and continuously adding water to said electrolyte at arestricted rate which maintains said electrolyte hydroxide solution andreplaces the water consummed in the cell by the chemical andelectrochemical reactions therein while electrical power issimultaneously withdrawn from between said anode and said cathode. 9.The method of claim 8 wherein said anode is sodium and said electrolytehydroxide is sodium hydroxide and wherein said operating temperature ismaintained above 12.3*C and below 97.5*C and said sodium hydroxideelectrolyte solution is maintained above 52 and below 75 percent byweight.
 10. The method of claim 8 wherein said anode is sodium and saidelectrolyte hydroxide is sodium hydroxide and wherein said operatingtemperature is maintained above 65*C and below 97.5*C and said sodiumhydroxide electrolyte solution is maintained at approximately 70 percentby weight.
 11. The method of claim 8 and including adding water to saidelectrolyte by passing water through a porous-walled container at arestricted rate.
 12. An electrochemical cell comprising: a. a solidelemental alkali metal consummable anode which is higly reactive withwater; b. a non-reactive electrically conductive cathode spaced fromsaid anode; c. an electrolyte in which said alkali metal anode and saidcathode are commonly directly immersed as a static cell; d. said cathodebeing shaped so as to contain said electrolyte; e. said cell providingan open and unimpeded liquid electrolyte circulation path between saidanode and said cathode; f. circuit connection means at said cathode andsaid anode for drawing electrical power from said cell; g. saidelectrolyte in the operation of said cell consisting essentially of aliquid solution in water of the hydroxides of said alkali metal anode;h. said electrolyte having a liquification temperature substantiallybelow the melting point of said alkali metal anode; i. said hydroxidesolution of said electrolyte being of sufficiently high concentration torestrict the direct chemical reaction rate between said alkali metalanode and the water in said electrolyte at said alkali metal anode toprevent both melting of said anode and blowing away of said electrolyte;and j. electrolyte molarity control means for continuously controllingsaid hydroxide solution concentration of said electrolyte as said cellis operated to maintain said electrolyte at a temperature below themelting point of said alkali metal anode and not in excess of theboiling point of said electrolyte.
 13. The electrochemical cell of claim12 wherein said control means comprises water feed control means forcontinuously feeding additional water into said electrolyte at acontrolled restricted rate in a manner whereby a portion of said waterdiffuses through said electrolyte to said cathode to be reducedelectrochemically at said cathode as electrical power is drawn from saidcell by said circuit connection means.
 14. The electrochemical cell ofclaim 13 wherein said anode and said cathode each have a substantiallyspiral configuration so that said spiral anode is positioned within saidspiral cathode.
 15. The electrochemical cell of claim 7 wherein saidcathode is an open-mesh metallic screen.